This invention relates to a communications network for general purpose data communications in a heterogeneous environment.
Recently, there has been a trend interconnecting a large, heterogeneous group of computers, batch terminals, and conversational terminals to form a general purpose network of computing recources. For instance, the computing resources of many governmental agencies and universities consist of a large variety of computers supporting a diverse user population with many different applications. Increasingly, these applications have come to require the resources of more than one computer. This has resulted in the direct point-to-point connection of rather large subsets of computers. As this type of network continues to grow, however, the communications plant must expand exponentially and the manpower required to modify existing computer operating systems to resolve differences in character sets and protocol becomes prohibitive.
In addition, data bases have become distributed resulting in enormous quantities of magnetic tape being hand-carried between machines where no point-to-point connections exist. The bandwidth of a cart full of tapes is impressive until one considers the amount of useful data on each tape in contrast to the time it takes to pull these tapes together and then disperse them.
Moreover, there is a growing use of terminals for remote job entry and interactive processing poses additonal problems. Often several terminals sit side by side when one would be sufficient. Unfortunately, each has been designed or programmed for only one manufacturer's computer. A similar situation occurs when the computers are the same, but no mechanism exists for switching a terminal between them.
In the prior art, attempts to connect more than two processors together for communications to form a processing system have met with considerable difficulty because of the large number of physical connections required. In addition, the drawbacks with regards to the timing problems involved in keeping the various processors from interfering with one another were considerable. As a partial solution, serial loop communications were developed whereby a single cable is utilized to connect a plurality of processors and terminal devices, such as storage units and input/output devices in series. It is common practice in these serial loop communications to utilize fixed length time slots. Each processor or terminal is provided with a permanently assigned time slot. This assigned slot is used to send information from a central control station to the processor or terminal. Similarly, the assigned slot is used to transmit data to the central station. The disadvantage with the assigned slot technique is illustrated when a terminal is in the process of receiving data and has no data to send. The transmission capacity represented by the empty slot cannot be used by other terminals subsequent to the receiving terminal since the slots are preassigned. Moreover, the problems relating to timing still remain.
Therefore, it is a primary object of this invention to provide an improved serial loop communications system.
Another object of the present invention is to provide a communications system in which information limited slots are dedicated and undedicated upon demand.
A still further object of the present invention is to provide a serial loop communication system in which clock information for the system is also derived from the transmitted information.
A still further object of the present invention is to provide a serial loop communication system in which synchronization to the character is automatic.